Remote communication utilizing wireless equipment typically relies on radio frequency (RF) technology. One application of RF technology is in locating, identifying, and tracking objects, such as animals, inventory, and vehicles. RF identification (RFID) systems have been developed that facilitate monitoring of one or more remote objects.
As shown in FIG. 1, a basic RFID system 10 includes two components: a reader or interrogator 12, and a transponder (commonly called an RFID tag) 14. The interrogator 12 and RFID tag 14 include respective antenna circuits 16, 18. In operation, the interrogator 12 transmits through its antenna circuit 16 a radio frequency interrogation signal 20 to the antenna circuit 18 of the RFID tag 14. In response to receiving the interrogation signal 20, the RFID tag 14 produces a modulated radio frequency signal 22 that is reflected back to the interrogator 12 through the tag antenna 18 by a process known as continuous wave backscatter.
The substantial advantage of RFID systems is the non-contact, non-line-of-sight capability of the technology. The interrogator 12 emits the interrogation signal 20 with a range from one inch to one hundred feet or more, depending upon its power output and the radio frequency used. Tags can be read through a variety of substances such as odor, fog, ice, paint, dirt, and other visually and environmentally challenging conditions where bar codes or other optically-read technologies would be useless. RFID tags can also be read at high speeds, in most cases responding in less than one hundred milliseconds.
A typical RFID system 10 often contains a number of RFID tags 14 and the interrogator 12. RFID tags are divided into three main categories. These categories are beam-powered passive tags, battery-powered semi-passive tags, and active tags. Each operates in different ways.
The beam-powered RFID tag is often referred to as a passive device because it derives the energy needed for its operation from the interrogation signal beamed at it. The tag rectifies the energy field and changes the reflective characteristics of the tag itself, creating a change in reflectivity that is seen at the interrogator. A battery-powered semi-passive RFID tag operates in a similar fashion, modulating its RF cross-section in order to reflect a delta to the interrogator to develop a communication link. Here, the battery is the source of the tag's operational power for optional circuitry. The passive and semi-passive devices, or non-active devices, reflect the energy from the interrogation signal. In contrast, in an active RFID tag, a transmitter is used to generate its own radio frequency energy powered by the battery.
The range of communication for such tags varies according to the transmission power of the interrogator 12 and the RFID tag 14. Battery-powered tags operating at 2,450 MHz have traditionally been limited to less than ten meters in range. However, devices with sufficient power can reach up to 200 meters in range, depending on the frequency and environmental characteristics.
Spread-spectrum RFID systems utilize frequency-chipping transmitters that simultaneously generate a band of frequencies. One such system is disclosed in U.S. Pat. No. 5,974,078 by Tuttle et al. Because this system relies on frequency chipping, it is difficult to implement with passive backscatter RFID tags, instead requiring the use of powered RFID tags that generate their own radio-frequency signals.
In U.S. Pat. No. 5,828,693 issued to Mays et al., a spread spectrum system using a homodyne receiver to receive the reflected signals is disclosed. In order to avoid interference inherent with homodyne receivers, Mays et al. requires the use of a complicated detector circuit for detecting interference and a controller that operates to vary the frequency of the interrogation signal to avoid the detected interference. Moreover, this system dynamically adjusts the frequency only in response to interference. Mays et al. also requires the use of pre-defined frequency scanning envelopes to ensure compliance with federal regulations that limit power levels.